Dynamic call bug

Development Standard Library
1 
protocoal Base {
 func fly()
}

extension Base {
 func fly() {
  print("base fly")
 }
  func testFly() {
   fly()
 }
}

class Animal: Base {
}

class Bird: Animal {
 func fly() {
  print("Bird fly")
 }
}

let t1 = Animal()
let t2 = Bird()
t1.testFly() // fly
t2.testFly() // fly
t2.fly() // Bird fly

usual, when call t2.testFly(), i want get "Bird fly"''s result. how can i do?

2

This is SR-103.

The workaround is to provide an implementation in the base class (here, Animal).